Evidence of High Harmonics from Echo-Enabled Harmonic Generation for Seeding X-Ray Free Electron Lasers

Echo-enabled harmonic generation free electron lasers hold great promise for the generation of fully coherent radiation in x-ray wavelengths. Here we report the first evidence of high harmonics from the echo-enabled harmonic generation technique in the realistic scenario where the laser energy modulation is comparable to the beam slice energy spread. In this experiment, coherent radiation at the seventh harmonic of the second seed laser is generated when the energy modulation amplitude is about 2–3 times the slice energy spread. The experiment confirms the underlying physics of echo-enabled harmonic generation and may have a strong impact on emerging seeded x-ray free electron lasers that are capable of generating laserlike x rays which will advance many areas of science.

Figure 1

Schematic of the EEHG experiment at SLAC. Two transverse cavities (TCAV1 and TCAV2) were installed to control the beam slice energy spread and measure the laser energy modulation to benchmark EEHG theory.

Figure 2

(a) COTR intensity vs wave plate angle; (b)–(d) are the time-resolved COTR intensities with $R_{56}^{(2)}$ set to 3.2, 5.2, and 6.2 mm, respectively.

Figure 3

227 nm coherent radiation intensity vs TCAV1 voltage when only the 1590 nm laser is on.

Figure 4

Spectrum of the radiation for various beam slice energy spreads: (a) below 1; (b) 2.2; (c) 4.4; (d),(e) 6.6 keV. The beam slice energy spreads are varied by changing TCAV1 voltages. (a)–(d) are the HGHG signals when only the 1590 nm laser is on, and (e) is the EEHG signal with both lasers on.